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Face masks are a crucial first line of defense against the spread of Covid-19, but that doesn’t mean they’re all created equal.
Cloth and N95 masks with filtration valves may be more comfortable for the wearer, but new study shows just how weak their defense is when it comes to preventing airflow and the spread of respiratory particles .
Using a mannequin and himself as a model, NIST research engineer Matthew Staymates uses fluid dynamics visualization to prove once and for all why these masks need to be tossed by the wayside.
The results were published Tuesday in the journal Fluid physics.
“If I wear a mask with a valve on it, I’m not helping.”
In April, the United States Center for Disease Control and Prevention began recommending that people wear sheet masks or surgical masks to slow the spread of Covid-19. It came a month after the World Health Organization declared the virus a pandemic.
Seven months later, mask wearing is a controversial issue in the United States, but the number of mask wearers has increased significantly since the CDC’s original recommendation.
The beauty of wearing a face mask is that it is a unique punch that protects both the wearer and anyone they interact with by limiting the flow of potentially contagious particles through surgical tissue or them. layers of fabric. But when it comes to valve masks, Staymates says in a statement that they only fulfill part of that equation.
“I don’t wear a mask to protect myself. I wear it to protect my neighbor because I could be asymptomatic and spread the virus without even knowing it,” Staymates said. “But if I wear a mask with a valve on it, I’m not helping.”
The problem with valves is that they were originally designed to be worn in situations where only wearer protection was important – like when sanding wood, for example – and to limit incoming particles without filtering out the air coming out.
That’s not exactly news – in a mask-wearing FAQ on its website, the CDC recommends that N95 masks with ventilators be re-covered with a surgical mask to prevent the spread of the virus. But Staymates explains in his article that he wanted to find a way to translate this crucial information to the public that was clear and acceptable.
So he decided to build a mannequin.
A mannequin in a mask – To show the difference between a standard N95 mask, an N95 mask with a valve, and no mask at all, Staymates concocted two different test scenarios.
For the first, he took a mannequin head and mounted a clear tube through his head and mouth to approximate the average exhalation airflow of an adult male. On these mannequins, Staymate simulated the three mask variations and recorded them by “taking” a few deep breaths.
In the second, less complex scenario, Staymates himself donned both an N95 mask with and without a valve.
In both scenarios, Staymates used fluid dynamic modeling and specialized lighting to better watch the airflow through the different mask scenarios. Videos from both scenarios show large waves of air spewing out of the open vent of the valve N95 masks, while masks without it appear to emit a much smaller, scattered bloom of air.
“When you compare the videos side by side, the difference is striking,” Staymates said. “These videos show how the valves allow air to exit the mask without filtering it, which defeats the purpose of the mask.”
Using computer modeling, Staymates was able to estimate the number of droplets in the air based on the density of bright pixels in the video and found that valve-less N95s resulted in a 95% decrease in pixel density (analog in this case to the droplet density.) while the valve N95s only reduced the pixel density by 40%.
In the article, Staymates even attributes this limited reduction in droplet density to particles simply stuck in the valve structure itself and not necessarily because of the intended filtration.
To take away – While these results are striking, a mannequin and a single human participant are far from a hermetic experience. Nonetheless, Staymates argues in the article that the point is still relevant today and suggests that future research may incorporate other varieties of masks as well as a 3D assessment of airflow.
In the meantime, he hopes these videos will help audiences visualize and understand the impact of what may appear to be just a small difference in design.
“[T]The main goal here is to create compelling visuals that are easy to understand and accessible to a wide audience, “says Staymates in the newspaper.” In addition, this work can help to raise awareness and perceive the usefulness of masks and masks.
Abstract: This work demonstrates the qualitative fluid flow characteristics of a standard N95 respirator with and without an exhalation valve. Schlieren imaging was used to compare an adult male breathing through an N95 respirator with and without a valve. The schlieren imaging technique showed the flow of hot air passing through these respirators, but did not provide information on the penetration of the droplets. For this, strategic illumination of the fog droplets was used with a mannequin head to visualize the penetration of the droplets through the two masks. The manikin exhaled with a realistic flow and speed that corresponds to an adult male. The penetration of mist droplets was also visualized with a custom system that seals each respirator to the end of a flow tube. The results of these qualitative experiments show that an N95 respirator without an exhalation valve is effective in preventing most droplets from entering through the mask material. The results also suggest that N95 respirators with exhalation valves are not suitable as a source control strategy to reduce the proliferation of infectious diseases that spread via respiratory droplets.
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